In the art of dentistry, casting abutments for receiving a dental prosthesis have become widely known. A structure (dental prosthesis) modeled after the natural tooth is mounted on the occlusal portion of the casting abutment. This structure is then mechanically attached to a dental implant, in most cases similar to a dowel pin, which was previously implanted in a patient's jaw bone.
As is well-known, such structures are manufactured as follows. First, a wax model is modeled on the casting abutment according to the shape of the natural tooth to be replaced. In most cases, the wax model is produced using a modeling aid made of plastics and plugged onto the casting abutment, which facilitates machining and keeps the access channel for the occlusal screw free. At the end of modeling, a structure made of wax has been created on the casting abutment.
Subsequently, the casting abutment with the wax structure modeled on top of it is embedded in an encapsulant. During heating of the mold, which is necessary for the casting process, the wax of the dental model and the modeling aid evaporate without leaving any residue. Thus, the negative mold is created which is later filled with a precious metal, such as gold, or a precious metal alloy (in the following termed precious metal for the sake of simplicity). In this way, a metal model is created which corresponds to the wax structure modeled on top of the casting abutment (in the following called casting model for the sake of simplicity).
The above method is called lost wax method, as is well-known in the art.
After the casting model has been created, it is normally veneered with a ceramic layer in order to achieve as realistic as possible a reproduction of the tooth to be replaced.
It has turned out that during manufacturing of the casting model by means of the conventional lost wax method, the precious metal can overflow under the collar area of the casting abutment, which would lead to undesired effects such as casting blisters. The overflow is due to the fact that surfaces of the casting abutment have not been sufficiently cleaned. Casting defects can lead to an imprecise fit or formation of a gap between casting abutment and dental implant. Such faulty casting models cannot be used, due to the high demands made on a precise connection between the casting abutment and the dental implant and the long-term demands made on dental prostheses, and must be re-manufactured.
In addition, with straight running collar areas of casting abutments (i.e. collar areas running vertically to the longitudinal axis of the casting abutment), it is especially problematic that these abutments have a tendency towards frequent overflow or excess of the molten precious metal, which has been heated up to high temperatures and cast. This is due, among others, to unfavorable flow characteristics of the molten precious metal on the collar area of the casting abutment.
As a remedy to the above-mentioned problem, normally a layer of graphite is applied in those areas where it is desired to stop the flow of the molten precious metal. The disadvantage of this procedure, however, is that graphite particles can contaminate the precious metal in the contact areas between the precious metal and the area covered by graphite.
Therefore, it is desirable to provide an improved casting abutment which can offer a perfect connection between the casting model and the dental implant even after casting. This is of special importance since the high compressive forces generated during chewing place immensely high requirements on the materials used and the precise fit which form the connection between casting model and dental implant. Minor errors during model manufacturing and during the gating process can lead to a metal model of the dental prosthesis which is no longer usable or must be refinished at considerable effort. Any cracks, contaminations, gaps or bruises must be avoided as well since they impair the mechanical long-term strength.